1. Field of the Invention
This invention relates to a matrix substrate, to a liquid crystal apparatus comprising such a matrix substrate for displaying pictures and characters and also to a display apparatus comprising such a liquid crystal apparatus. It also relates to a method of manufacturing such a matrix substrate and such a liquid crystal apparatus.
2. Related Background Art
The current world has entered a so-called multi-media age and devices and equipment for exchanging pictorial information for communication purposes are playing an ever-increasingly important role in the world. In this tide, liquid crystal apparatus are attracting attention because they can be realized in a thin and portable form and consume only little power so that the industry of manufacturing such devices has grown as one of the major industries that is comparable with the semiconductor industry.
Liquid crystal apparatus are at present popularly used for so-called notebook size personal computers having a size as short as ten inches. Additionally, liquid crystal is expected to play a major role in displays of electronic work stations and home television sets that typically comprise a large display screen. However, a large display screen involves high manufacturing cost and is required to meet rigorous electric requirements for driving the large screen. As a rule of thumb, the manufacturing cost of a liquid crystal display panel is said to be proportional to the square or cube of the size of the screen.
Thus, as an alternative, a projection system designed to use a small liquid crystal panel and enlarge the image formed on it has been proposed. Such a system is made feasible mainly due to the recent development of finely sized semiconductor devices that operate excellently and can be manufactured at low cost. In view of the recent technological development, there is an increasing demand for small TFTs provided with a satisfactory drive power to be used in liquid crystal display panels comprising TFTs that use thin film transistors as switching devices for pixel electrodes. Additionally, TFTs using polycrystalline Si rather then amorphous Si are getting popularity. Video signals for the level of resolution meeting the NTSC standards that are used for the NTSC television system do not require high speed processing capabilities.
Therefore, not only TFTs but also components of peripheral circuits including shift registers and decoders may be made of polycrystalline Si to produce a liquid crystal display apparatus wherein a display region and peripheral drive circuits are integrally formed. However, polycrystalline Si is less performing than monocrystalline Si so that shift registers, for example, may have to be divided into a plurality of groups that are installed separately in order to realize a television set with a level of resolution higher than the level required by the NTSC standards or a computer display with the so-called XGA (extended graphics array) or SXGA (super extended graphics array) class of resolution. Then, noise can appear as ghost along the boundaries of the separated devices in the display region to provide a problem to be solved.
In an attempt to bypass this problem, display apparatus comprising a monocrystalline Si substrate having a high drive potential have been proposed to replace display apparatus of the above described integral type using polycrystalline Si. Since the drive potential of the transistors of the peripheral drive circuits of such display apparatus is satisfactory, the above described technique of dividing devices is not necessary here. Consequently, the S/N ratio of the lines connecting the display apparatus and the peripheral drive circuits has a large value and hence the noise problem can be successfully avoided.
Regardless of polycrystalline or monocrystalline Si, a reflection type liquid crystal apparatus comprising reflection type liquid crystal devices can be prepared by connecting the drain of the switching device of each pixel and the reflection electrode and arranging liquid crystal between the reflection electrode and a transparent common electrode.
A reflection type liquid crystal apparatus is adapted to display an image by means of rays of light reflected by the pixel electrodes of the apparatus so that it can effectively utilize light if compared with a transmission type liquid crystal apparatus that has to suppress light entering the semiconductor layer of switching devices while showing a large aperture to shield ratio.
On the other hand, the reflection type liquid crystal apparatus is required to have the pixel electrodes show a large reflectivity, which represents a requirement from which the transmission type liquid crystal apparatus is exempted. Japanese Patent Application Laid-Open No. 8-179377 (JPA 8-179377) is directed to this problem and proposes a technique for polishing pixel electrodes and passivation film for a liquid crystal apparatus by means of chemical mechanical polishing (CMP). The proposed technique will be summarized here by referring to FIGS. 27A and 27B of the accompanying drawings.
Referring to FIG. 27A, passivation film 2011 and pixel electrodes 2009 are polished by means of a polishing agent containing an etchant adapted to etch both the passivation film 2011 and the pixel electrodes 2009 until the pixel electrodes 2009 and the passivation film 2011 become flush with each other and mirror shine. Then, since the pixel electrodes 2009 and the passivation film 2011 are flush with each other, an orientation film can be formed directly on the pixel electrodes 2009 and an evenly distributed electric field can be applied to the liquid crystal due to the flat and smooth surface of the pixel electrodes 2009. The above cited JPA 8-179377 document points out that the surface of a liquid crystal panel polished by CMP can become droopy in peripheral areas and proposes the use of dummy pixels arranged between the drive circuit and the display pixel area of the liquid crystal apparatus. This will be summarily described below by referring to FIGS. 28A through 28D of the accompanying drawings.
Referring firstly to FIG. 28A showing a reflection type active matrix display panel, it comprises a substrate 2001, a display pixel area 2017 and signal scan/drive circuits 2018 arranged in the peripheral area of the display panel to show steps on the panel surface because of the difference in the height (thickness) between the display pixel area 2017 and the signal scan/drive circuits 2018. When the passivation film (oxide film) 2011 covering the stepped sections is polished by means of a polishing agent containing etchant for etching the oxide film, then the film can become droopy in peripheral areas surrounding the display pixel area 2017 as shown in FIG. 28B.
In order to prevent such droopy film, dummy pixels 2019 are arranged between the display pixel area 2017 and the signal scan/drive circuits 2018 to surround the display pixel area 2017 as illustrated in FIG. 28C. With this arrangement, while the oxide film may droop down to some extent from the step separating the display pixel area 2017 and the signal scan/drive circuits 2018, it is held substantially flat within the display pixel area 2017 as illustrated in FIG. 28D.
Meanwhile, the applicant of the present patent application proposed a method of manufacturing a display apparatus by using CMP in Japanese Patent Application No. 8-178711.
According to the above identified patent document, there is provided a method of manufacturing a display apparatus comprising an active matrix substrate on which each pixel electrode is provided with a switching transistor, an electrode substrate disposed opposite to the active matrix substrate and liquid crystal held between the substrates, wherein said method comprises a process of forming pixel electrodes that includes a chemical mechanical polishing step.
The above identified patent application also proposes a method of manufacturing a display apparatus, wherein an insulation layer is patterned to produce grooves therein and then a layer of the material of pixel electrodes is formed by deposition both in the grooves and on the insulation layer so that a continuous plane surface is produced on both the insulation layer and the layer of the material of pixel electrodes when the layer of the material of pixel electrodes is polished by CMP. With this technique, the gaps separating the pixel electrodes are satisfactorily filled with the insulation layer to eliminate any undulations on the surface that can give rise to irregular reflection and defective orientation and make it possible to display high quality images.
A major difference between the invention of Japanese Patent Application Laid-Open No. 8-179377 (JPA 08-179377) as described earlier and that of Japanese Patent Application No. 8-178711 filed by the applicant of the present patent application is that a passivation film 2011 is formed after forming a layer of the material of pixel electrodes 2009 and then they are polished by CMP according to JPA 08-179377, whereas an insulation layer is patterned to produce grooves therein and then a layer of the material of pixel electrodes is formed by deposition both in the grooves and on the insulation layer so that a continuous plane surface is produced on both the insulation layer and the layer of the material of pixel electrodes when the layer of the material of pixel electrodes is polished by CMP according to Application No. 8-178711. While JPA 8-179377 proposes dummy pixels to be formed between the pixel area and the drive circuits, it cannot eliminate a droopy surface on the peripheral areas.
On the other hand, while Application No. 8-178711 provides an excellent technique comprising a step of patterning an insulation layer to produce a matrix of grooves for depositing the material of pixel electrodes therein, it has been found that it is difficult to uniformly produce grooves for forming pixel electrodes without giving due consideration to resist to be applied to the pixel display region and the remaining peripheral areas and the pitch and the depth of the grooves.
This will be described by referring to FIGS. 26A and 26B of the accompanying drawings.
FIGS. 26A illustrates a stage of manufacturing a display apparatus according to Application No. 8-178711, where there have been formed on a substrate 201 an insulation layer 211 by deposition and grooves 280 for forming pixel electrodes in said insulation layer 211 after forming a semiconductor layer (not shown) to be used for pixel switching devices and drive circuit sections 260 for driving the switching devices and now the material of pixel electrodes 213 is deposited both in the grooves 280 and on the insulation layer 211. In FIG. 26A, reference numerals 250 and 270 respectively denote a pixel display region and a sealing region for sealing the liquid crystal material by using the active matrix substrate and the opposite substrate (not shown). Referring to FIG. 27A, while typically several to tens of several micrometers wide grooves 280 have to be regularly formed for pixel electrodes in the pixel display region 250, grooves do not have to be regularly formed on the drive circuit regions 260 and the sealing regions 270 so that the resist arrangement may differ between the pixel display region 250 and the remaining areas including the drive circuit regions 260 and the sealing regions 270. In other words, the grooves 280 may show different and uneven depths as illustrated in FIG. 26A unless due consideration is paid to resist to be applied to the pixel display region and the remaining peripheral areas and the pitch and the depth of the grooves. FIG. 26B shows the surface condition after a CMP operation and it will be found that the display region 250 and the remaining areas including the drive circuit regions 260 and the sealing regions 270 show difference in the thickness of the insulation layer 211.